Energy evolution law and failure mechanism of coal-rock combined specimen

In order to study the energy evolution and failure mechanism of coal-rock combined structures during load- ing process,the uniaxial one-time loading and cyclic loading and unloading tests were carried out on coal,rock and three sets of coal-rock combined specimens. The evolution of the input energy density,elastic energy density,dissipated energy density,elastic modulus and uniaxial compressive strength of coal-rock combined specimens were evaluated, and the energy storage characteristics of different samples were obtained. Based on the mechanical response,the energy evolution and deformation and failure characteristics of coal-rock combined specimens,and the energy-driven mecha- nism of coal and rock combined specimen were established and discussed. The results show that the input energy density,elastic energy density and dissipated energy density of the coal-rock combined specimen in the pre-peak stage ex- hibit distinct nonlinear growth characteristics with the increase of the axial stress,the energy density-stress curve of the pre-peak stage can be divided into compaction section,elastic deformation section and unsteady fracture development stage,the ratio of elastic energy stored inside the sample is high,and the proportion of dissipated energy is small,after the sample reaches yield,the proportion of dissipated energy begins to increase. The uniaxial compressive strength and elastic modulus of coal-rock combined specimens are slightly greater than the pure coal and much lower than the rock specimens. The mechanical properties of the combined specimens increase slightly with the rock strength. This increase is limited due to the fact that the coal is the dominated factor controlling the mechanical properties of the combined strength. The energy-driven mechanism of coal-rock combination failure can be summarized as that during the loading process of the coal-rock combined specimen,the coal and the rock begin to store elastic energy continuously,the ener- gy storage speed of coal is fast,when the internal elastic energy reaches the energy storage limit,shear failure initially occurs within the coal,leading to a X-shaped conjugate slope. When the energy released by the destruction is transmit- ted to the rock and reaches the energy storage limit of the rock,the crack in the coal expands into the interior of the rock,causing a tensile failure of the rock.